1. Field of the Invention
The present invention relates to a fuel-injection method and an apparatus therefor, in which fuel delivered through a common rail is charged from injectors into combustion chambers, and further relates to a process for defining the beginning of pressure drop in the common rail, which is applicable to the common-rail, fuel-injection system.
2. Description of the Prior Art
In a typical fuel-injection control for engines such as diesel engines, a common-rail, fuel-injection system is conventionally known, in which the fuel injected is highly intensified in pressure and the fuel-injection characteristics such as a timing and quantity of fuel injected per cycle are adequately controlled in accordance with the engine operating conditions. On most common-rail, fuel-injection systems, the fuel pressurized at a preselected pressure is stored in a fuel supply line common to the injectors and the consequent stored fuel is injected from each injector into the associated combustion chamber. In order to inject the pressurized fuel at the individual fuel injector with the optimal fuel delivery condition for the engine operation, a controller unit regulates the fuel pressure in the common rail and the control valves each installed in the individual fuel injector.
A conventional common-rail, fuel-injection system will be explained below with reference to FIG. 7. The fuel delivery to injectors 1 is carried out through a common rail 2 and injection lines 3, each of which is a part of the fuel-flow line. A fuel feed pump 6 draws fuel from a fuel tank 4 through a fuel filter 5 and forces it under a preselected pressure to a fuel-supply pump 8 through a fuel line 7. The fuel-supply pump 8 is of, for example, a fuel-supply plunger pump driven by the engine, which intensifies the fuel to a high pressure determined depending on the engine operating conditions, and supplies the pressurized fuel into the common rail 2 through another fuel line 9. The fuel, thus supplied, is stored in the common rail 2 at the preselected high pressure and forced to the injectors 1 from the common rail 2. The injectors 1 are provided in compliance with the type of engines (the number of the cylinders) and controlled with a controller unit 12 of an electronic control unit to thereby inject the fuel, supplied from the common rail 2, to the associated combustion chambers with an adequate timing and metered quantity of fuel. The injection pressure of the fuel sprayed out of the injectors 1 is substantially equal with the pressure of fuel stored in the common rail 2, that is, the common rail pressure, which is thus regulated in order to control the injection pressure.
The fuel relieved from the fuel-supply pump 8 is allowed to flow back the fuel tank 4 through a fuel-return line 10. The unconsumed fuel remaining in each injector 1 out of the fuel fed through the injection lines 3 into the injectors 1 may return to the fuel tank 4 through a fuel-recovery line 11. The controller unit 12 is applied with various signals of sensors monitoring the engine operating conditions, such as a cylinder identifying sensor and a crankshaft position sensor for detecting the engine rpm Ne, an accelerator pedal sensor for detecting the depression Acc of an accelerator pedal, an engine coolant temperature sensor, an intake manifold pressure sensor and the like. The controller unit 12 may regulate the fuel-injection characteristics, that is, the injection timing and the quantity of fuel injected out of the injectors 1, depending on the applied signals, to thereby allow the engine to operate as fuel-efficient as possible. Moreover, the controller unit 12 is applied with a detected single as to a common-rail pressure reported from a pressure sensor 13 installed in the common rail 2. Injection of fuel out of the injectors 1 consumes the fuel in the common rail 2, causing the pressure drop in the common-rail pressure, at which the controller unit 12 regulates the discharge of the fuel-supply pump 8 so as to keep the common-rail pressure constant.
As described in Japanese Patent Publication No. 60020/1985, the prior common-rail, fuel-injection system controls the fuel-injection pressure to the desired value in accordance with the engine operating conditions, while calculating the fuel-injection characteristics, that is, the quantity of fuel metered to be injected, which quantity is defined by the pressure and duration for injection per cycle, and the timing of fuel injection, in compliance with the engine operating conditions, thereby achieving the fuel-injection characteristics optimal for the engine operating conditions. The common-rail pressure defining the injection pressure is intensified by the fuel-supply pump, while regulated to a desired injection pressure by means of a pressure regulator valve.
In the prior common-rail, fuel-injection system, the controller unit applies the command pulses for the injection command signals to the solenoid-operated valves, which are provided in the injectors, each to each injector. The solenoid-operated valves energized with the command pulses lift the needle valves to open the injection holes at the nozzle tips of the injectors 1, resulting in allowing the fuel charges into the combustion chambers. However, a time-lag is usually present, spanning from the time when the controller unit issues the command pulse to be signaled to the solenoid-operated valve to the time when the fuel is actually injected out of the nozzle holes of the injectors. Such time lag arises from a response delay inherent in the driving circuit, that is, a delay spanning from the time of signaling the command pulse from the controller unit to the solenoid to the time of the actual energization of the solenoid, and a mechanical delay in the injectors, during which the needle valve is made to lift after the energization of the solenoid to thereby allow the fuel injection out of the injector. Moreover, even if the timing when the command pulses issued from the controller unit turn "on" is precisely maintained at constant, every injector has tended to vary or scatter in the timing of the beginning of the fuel injection owing to characteristic difference in the individual injectors, aging or the like.
In most conventional fuel-injection systems dealing with the problem as described just above, the time lag is considered to be constant so that the scattering for every injector in the time lag is ignored. Accordingly, the optimal combustion of fuel can not be appreciated due to characteristic difference in the individual injectors, aging or the like. This results in the major problems in which the exhaust gas control becomes inferior and vibration occurs in the engine owing to the difference in combustion timing among the individual cylinders.
In contrast, disclosed in Japanese Patent Laid-Open No. 210174/1996 is a method of detecting fuel-injection timing and an apparatus therefor, which has for its object to determine accurately the fuel-injection timing in the diesel engines. According to this prior art, the fuel pressure is monitored at a fuel line connecting a fuel injection pump with fuel-injection nozzles, while a pressure drop greater than the preselected value is detected, which happens first after the monitored fuel pressure reaches a design high pressure. The initiation of the first pressure drop is identified as the timing of the beginning of the fuel injection.
Unlike the common-rail fuel-injection system, nevertheless, the method and apparatus for detecting fuel-injection timing, disclosed in the above publication, belong to a fuel-injection system including a fuel distributor pump to meter and direct fuel to the injectors, or an inline fuel injection pump.
On the other hand, Japanese Patent Laid-Open No. 47137/1998 discloses therein a method of detecting fuel-injection timing and an apparatus therefor in a common-rail, fuel-injection system. A pressure sensor in the common rail detects the timing when the pressure drop happens in the common rail pressure after the fuel injection out of the injectors. The actual timing of the beginning of the fuel injection is calculated, in compliance with the timing of pressure drop, by going backwards by the length of time during which the pressure waves are transmitted from the injection nozzles to the common rail. The deviation of the resultant actual timing from the desired timing of the beginning of the fuel injection is stored for compensating the desired timing of the beginning of the next fuel injection. That is to say, the above citation discloses the conception of compensating the desired timing of the start of the fuel injection with the detected timing of the start of pressure drop in the common rail pressure.
However, since the pressure drop in the common rail pressure due to the fuel injection is normally accompanied by pulsative waves, it is actually very hard to detect accurately the timing of the beginning of the pressure drop in the common rail pressure. In this regard, the above-cited Japanese Patent Laid-Open No. 47137/1998 discloses no teaching of the specific measures about how to detect the timing of the beginning of the pressure drop of the common rail pressure.
Based on the recognition that defining closely the timing of the beginning of pressure drop is critical for accurate control of the timing of the initiation of the fuel injection on the individual injectors in the common-rail, fuel-injection system, the inventors have already proposed a method of defining accurately the timing of the beginning of the pressure drop in the common rail pressure, which is explained in co-pending senior Japanese Patent Laid-Open No. 101149/1999, and further confirmed the results satisfactory to some degree. Nevertheless, there is still the room for improvement and, therefore, how to define strictly the timing of the beginning of pressure drop is the major subject for accurately controlling the timing of the initiation of the fuel injection on the individual injectors in the common-rail, fuel-injection system.